1. Field of Invention
This invention relates to core drilling apparatus which includes a mechanism for preventing flow of drilling mud through the inner tube of a core barrel. Specifically, this invention relates to closure mechanisms which register against a seat in the inner tube assembly in response to conditions existing in the drill string, and which are maintained in registration with or in close proximity to the seat under substantially all normally encountered drilling conditions.
2. State of the Art
Closure mechanisms associated with the inner tube of a core barrel which operate to prevent drilling mud or fluid from traveling down the inner tube are well known. In the field, drilling mud is pumped downwardly through the inner tube of a core barrel while the drill string is being run to the bottom of the well bore in order to prevent debris from entering the inner tube prior to commencement of the coring operation. Drilling fluid is continuously pumped through the inner tube until the drill bit reaches the bottom of the hole. Oftentimes, drilling mud is circulated for some time "on bottom" after the drill bit reaches bottom to ensure a clean inner tube prior to coring. Immediately before drilling begins, a mechanism, typically known as a drop ball mechanism, is activated in the inner tube assembly to close off the central bore of the inner tube. The drilling fluid is then diverted or rerouted to and through the annular space formed between the inner tube and outer barrel of the core barrel. The fluid is then directed through nozzles or other apertures which are in the core bit crown.
In some core drilling systems, a seat in provided in the upper portion of the inner tube assembly and a ball is dropped from the surface through the drill string to the core barrel, eventually coming into registration with the seat to close off the central bore of the core barrel above the inner tube. In other core drilling systems, a drop ball mechanism is positioned in the drill string, usually in a subsection (also referred to as a "sub") of the drill string. In response to a stimulus from the surface, such as increased fluid flow rate, the ball is released to drop down until it comes into registration with the seat.
The aforementioned closure mechanisms have limited utility and effectiveness in certain situations. For example, prior art closure mechanisms are not configured to maintain the ball against the seat when flow of drilling mud is stopped. The ball may drop away from the seat when drilling takes places horizontally, or when changes in pressure arising in the inner tube cause the ball to rise momentarily from the seat. It has been shown that in some coring situations, especially high angle or horizontal, once dislodged, the ball may have trouble seating again, allowing mud pressure to damage the core. Further, when a motor is being used downhole to rotate the drill bit the motor will obstruct access to the core barrel below the motor in the drill string, making many prior art ball dropping mechanisms impractical or impossible to use in such drill string configurations.
Therefore, it would be an improvement in the art to provide a self-contained closure mechanism associated with the inner tube assembly of a core barrel which is activated in response to conditions existing within the core barrel and which maintains the ball in immediate proximity to the seat under all drilling conditions. Such a mechanism also would be useful and highly desirable for utilization in other downhole applications where dropping a ball from the surface or from a sub in the drill string is undesirable or even impossible. Such applications include motor coring, coring with certain MWD (Measurement While Drilling) or other electronic devices, or turbine coring.